A wireless communication system generally uses intermediate frequency (IF) conversion during signal transmission to convert a transmission signal into a signal having a carrier frequency for transmission or into a baseband signal for reception. However, recently, a direct conversion method has widely been used without IF conversion due to costs, etc.
Direction conversion receivers illustrated in FIGS. 1 and 2 have a very simple topology form in which a mixer 140 mixes a received signal with a local signal of the same frequency as the received signal so that a central frequency of a signal can be located in a DC, and a low pass filter 120 selects a channel. A radio frequency (RF) signal received from an antenna is amplified to a low-noise signal while passing through a low-noise amplifier (LNA) 110.
When using direct conversion, if an RF received by a local oscillator (LO) 130 for generating a frequency identical to a carrier frequency is the same as an RF received on air, an unnecessary component is generated in a DC part in terms of hardware. This is called DC offset.
FIG. 1 illustrates DC offset generated by a leakage component of the local oscillator 130. FIG. 2 illustrates DC offset generated by an interference signal having the same frequency as a carrier frequency.
In direct conversion, an RF signal is directly converted into a baseband signal through the local oscillator 130. In this conversion process, if a leakage component of the local oscillator 130 is generated or the same frequency as RF is added without filtering, a DC signal, a frequency of which is 0 Hz, caused by the same frequency, may be self-mixed. Such a DC offset component serves as a serious obstacle to readout of a baseband signal and deteriorates a signal-to-noise ratio (SNR). Accordingly, coping with the DC offset is a difficult problem to overcome in direction conversion.
To solve such a problem, in pulse mode communication employing a time slot, such as a global system for mobile communications (GSM), a method is used for discharging a DC charge in a time interval during which communication is not performed. In code division multiple access (CDMA), a self-calibration method is used due to a complicated modulation scheme. In OFDM-based communication, the DC offset problem is solved such that no signal is carried on a subcarrier corresponding to DC, that is, by including ‘0’ in the subcarrier. However, in DFT-S OFDMA, since DC offset is reduced to some degree through DFT-despreading, no particular process is performed for transmission. In uplink of 3rd generation partnership protocol (3GPP) long term evolution (LTE) to which single carrier (SC)-FDMA, which is a sort of DFT-S OFDMA, is applied, half of spacing between data subcarriers is shifted to reduce an influence caused by the DC offset. According to specifications up to now, spacing between uplink subcarriers is 15 KHz and 7.5 KHz is shifted during transmission.
FIG. 3 illustrates a conventional method for preventing DC distortion.
In 3GPP LTE for example, the first SC-FDMA symbol is transmitted as a time-continuous signal as indicated by the following Equation 1:
                                          s            l                    ⁡                      (            t            )                          =                              ∑                          k              =                              -                                  ⌊                                                            N                      RB                      UL                                        ⁢                                                                  N                        SC                        RB                                            /                      2                                                        ⌋                                                                                    ⌈                                                      N                    RB                    UL                                    ⁢                                                            N                      SC                      RB                                        /                    2                                                  ⌉                            -              1                                ⁢                                          ⁢                                    a                                                k                                      (                    -                    )                                                  ,                l                                      ·                          ⅇ                                                j2π                  ⁡                                      (                                          k                      +                                              1                        /                        2                                                              )                                                  ⁢                Δ                ⁢                                                                  ⁢                                  f                  ⁡                                      (                                          t                      -                                                                        N                                                      CP                            ,                            l                                                                          ⁢                                                  T                          S                                                                                      )                                                                                                          [                  Equation          ⁢                                          ⁢          1                ]            
where 0≦t<(NCP,l+N)×Ts, k(−)=k+└NRBULNSCRB/2┘, N=2048, and Δf=15 kHz. In Equation 1,
      1    2    ⁢  Δ  ⁢          ⁢  findicates an operation shifting half of spacing between subcarriers. In the above example,
      1    2    ⁢  Δ  ⁢          ⁢  fmeans a 7.5-KHz shift.
To prevent DC distortion of data of uplink SC-FDMA in LTE, the above frequency shift method is used to shift all uplink channels by an absolute value of 7.5 KHz.
Referring to FIG. 3, a subcarrier 310 before shift is partially overlapped with a DC subcarrier position, while a subcarrier 320 after shift is not overlapped with the DC subcarrier position.
Meanwhile, a random access channel (RACH) is a channel used for a mobile station to obtain initial uplink synchronization. When a mobile station is turned on or enters an active mode from a long-time idle mode, the RACH is used to reset the uplink synchronization. The RACH can be used without adjusting time or frequency synchronization.
A time-continuous random access signal using 6 resource blocks, that is, 72 subcarriers in LTE is transmitted as indicated by the following Equation 2.
                              s          ⁡                      (            t            )                          =                              β            PRACH                    ⁢                                    ∑                              k                =                0                                                              N                  ZC                                -                1                                      ⁢                                          ∑                                  n                  =                  0                                                                      N                    ZC                                    -                  1                                            ⁢                                                                    x                                          u                      ,                      v                                                        ⁡                                      (                    n                    )                                                  ·                                  ⅇ                                                            -                      j                                        ⁢                                                                  2                        ⁢                        π                        ⁢                                                                                                  ⁢                        nk                                                                    N                        ZC                                                                                            ·                                  ⅇ                                                            j2π                      ⁡                                              (                                                  k                          +                          φ                          +                                                      K                            ⁡                                                          (                                                                                                k                                  0                                                                +                                                                  1                                  /                                  2                                                                                            )                                                                                                      )                                                              ⁢                    Δ                    ⁢                                                                                  ⁢                                                                  f                        RA                                            ⁡                                              (                                                  t                          -                                                      T                            CP                                                                          )                                                                                                                                                                              ⁢                  [                      Equation            ⁢                                                  ⁢            2                    ]                    
where 0≦TPRE+TCP and k0=kRANSCRB−NRBULNSCRB/2. In Equation 2, βPRACH denotes an amplitude scaling factor. A position in a frequency domain is controlled by kRA (0≦kRA≦NRBUL−6). K=Δf/ΔfRA denotes a difference between spacing between random access preamble subcarriers and spacing between uplink data subcarriers. ΔfRA denotes spacing between the random access preamble subcarriers and φ denotes a fixed offset of a frequency domain of the random access preamble within resource blocks. ΔfRA and φ are defined as follows.
TABLE 1Frame structureBurst formatΔfRAφType 10-31250 Hz12Type 207500 Hz211875 Hz9
In Equation 2,
      ∑          n      =      0                      N        ZC            -      1        ⁢                    x                  u          ,          v                    ⁡              (        n        )              ·          ⅇ                        -          j                ⁢                              2            ⁢            π            ⁢                                                  ⁢            nk                                N            ZC                              indicates a DET-S operation of a Zadoff-Chu (ZC) sequence xu,v(n), and
      ∑          k      =      0                      N        ZC            -      1        ⁢            (      .      )        ·          ⅇ                        j2π          ⁡                      (                          k              +              φ              +                              K                ⁡                                  (                                                            k                      0                                        +                                          1                      /                      2                                                        )                                                      )                          ⁢        Δ        ⁢                                  ⁢                              f            RA                    ⁡                      (                          t              -                              T                CP                                      )                              indicates a term related to time-domain signal conversion through inverse discrete Fourier transform (IDFT) or inverse fast Fourier transform (IFFT) and to frequency shift.
Δf is 15 KHz, while ΔfRA becomes 1250 Hz, 7500 Hz, or 1875 Hz according to a frame structure type and a burst format.
FIG. 4 illustrates a process of shifting a random access preamble subcarrier according to the conventional method for preventing DC distortion.
When half of spacing between subcarriers, for example, 7.5 KHz can be divided by ΔfRA of 1250 Hz, 7500 Hz, or 1875 Hz, even though
      1    2    ⁢  Δ  ⁢          ⁢  fis shifted with respect to RACH, an RACH preamble may be transmitted at a position 410 of a DC subcarrier. Accordingly, a DC offset problem is not solved. In case of RACH having spacing of 1250 Hz between subcarriers, data avoids DC distortion using a frequency shifted by
      1    2    ⁢  Δ  ⁢          ⁢  fof 7.5 KHz, while RACH uses a frequency domain with narrow spacing of ΔfRA of 1250 Hz based on the frequency position shifted by
      1    2    ⁢  Δ  ⁢          ⁢  fof 7.5 KHz. In this case, since 7.5 KHz is a multiple of ΔfRA, a subcarrier of a specific RACH uses the DC subcarrier position.
Even though ΔfRA is 7500 Hz or 1875 Hz, since Δf is a multiple of ΔfRA, the same phenomenon occurs.